In recent years, there have been great advancements in the speed, power, and complexity of integrated circuits. Large scale integrated circuits comprise thousands of devices placed on a single integrated circuit chip. A standard process for electrically isolating the devices on an integrated circuit chip is the local oxidation of silicon process (LOCOS process).
The creation of a prior art LOCOS isolation structure is illustrated in FIG. 1 and in FIG. 2. The structure 100 shown in FIG. 1 comprises a silicon substrate 110. A layer of pad oxide 120 made up of silicon dioxide (SiO2) is placed on the silicon substrate 110. Then a layer 130 of silicon nitride (Si3N4) is placed on the layer pad oxide 120. A mask and etch procedure is used to etch an aperture 140 through the silicon nitride layer 130 and through the pad oxide layer 120 down to the silicon substrate 110. The application of the mask and etch procedure creates the structure 100 shown in FIG. 1.
Then the portions of the silicon substrate 110 that are exposed through aperture 140 are exposed to steam (H2, O2) at a relatively high temperature (e.g., one thousand degrees Celsius (1000° C.)). The oxygen in the steam oxidizes the silicon substrate 110 to form silicon dioxide (SiO2). The oxidation process causes the oxidized portion of the silicon substrate 110 to increase in size.
The resulting structure 200 is shown in FIG. 2. The oxidized portion of the silicon substrate 110 is designated with reference numeral 210. As shown in FIG. 2, the oxidized portion 210 of the silicon substrate 110 is sometimes referred to as a field oxide. The field oxide 210 forms an isolation structure or isolation barrier that electrically separates and isolates portions of the integrated circuit chip.
At the edges of the field oxide 210 (near the edges of the silicon nitride portions 130) the thickness of the field oxide tapers off. The maximum thickness of the field oxide 210 (shown by double arrows in FIG. 2) gradually decreases near edges of the field oxide 210 and tapers down to the thickness of the pad oxide 120.
The tapering profile of the edges of the field oxide 210 forms a portion of the field oxide 210 that is known as a “bird's beak.” The bird's beak portion of the field oxide 210 in FIG. 2 is designated with reference numeral 220.
If the bird's peak portion of the field oxide has a relatively graded slow tapering profile the resulting bird's beak profile will have the bird's peak profile 300 shown in FIG. 3. For convenience in description the bird's beak profile 300 will be referred to as a “graded” bird's beak.
If the bird's beak portion of the field oxide has a relatively short quick tapering profile the resulting bird's beak profile will have the bird's beak profile 400 shown in FIG. 4. For convenience in description the bird's beak profile 400 will be referred to as an “abrupt” bird's beak.
In prior art manufacturing processes a thick pad oxide and/or a thin silicon nitride layer will create a graded bird's beak 300. Similarly, in prior art manufacturing processes a thin pad oxide and/or a thick silicon nitride layer will create an abrupt bird's beak 400.
A significant advantage of the abrupt bird's beak is that the abrupt bird's beak takes up less lateral space than a graded bird's beak. This means that there is less space required to form the field oxide isolation structure. Therefore there is more space remaining in the integrated circuit chip for the integrated circuit devices (e.g., transistors). This concept is usually expressed by stating that the abrupt bird's beak provides a better packing density for the integrated circuit devices. A major drawback of the abrupt bird's beak is that the abrupt bird's beak has a lower breakdown voltage.
Conversely, a major advantage of the graded bird's beak is that it provides a higher breakdown voltage. But the graded bird's beak takes up more lateral space than an abrupt bird's beak. This means that the graded bird's beak has a correspondingly worse packing density in the integrated circuit chip for the integrated circuit devices (e.g., transistors).
In an integrated circuit device that is isolated by a LOCOS process it is well known that the bird's beak is one of the areas that has a high value of impact ionization. It is also well known that the presence of increased impact ionization reduces the breakdown voltage of the integrated circuit device. Therefore, in order to increase the breakdown voltage of a LOCOS isolated device, it would be desirable to decrease the amount of impact ionization in the LOCOS isolated device.
There is a need in the art for an efficient method for manufacturing an integrated circuit that has an increased breakdown voltage. In particular, there is a need in the art for a method that is capable of increasing a breakdown voltage in a LOCOS isolated device by reducing the amount of impact ionization that is present in a bird's beak of the device.
The present invention provides an efficient method for increasing the breakdown voltage in a LOCOS isolation integrated circuit device. The method comprises forming a portion of a field oxide in the integrated circuit so that the field oxide has a gradual profile. The gradual profile of the field oxide reduces the value of impact ionization in the field oxide by creating a reduced value of electric field for a given value of applied voltage. The reduction in the value of impact ionization increases the breakdown voltage of the integrated circuit.
As will be described more fully below, an advantageous embodiment of the method of the invention forms the gradual profile of the portion of the field oxide by using an increased thickness of pad oxide and a reduced thickness of silicon nitride during the field oxide oxidation process that creates the field oxide.
It is an object of the present invention to provide a method for efficiently manufacturing an integrated circuit that has an increased breakdown voltage.
It is an object of the present invention to provide a manufacturing method for an integrated circuit that increases a breakdown voltage in a LOCOS isolated device by reducing the amount of impact ionization that is present in a bird's beak of the LOCOS isolated device.
It is another object of the present invention to provide a manufacturing method for an integrated circuit that creates a gradual profile in a bird's beak field oxide in an area where it is important to have a high breakdown voltage.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those persons who are skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Persons who are skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Persons who are skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
Definitions for certain words and phrases are provided throughout this patent document, those persons of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as future uses, of such defined words and phrases.